Lymphatic & Immune Systems: Structure and Function

Lymphatic and Immune System Overview

Introduction

The lymphatic and immune systems are essential for fluid balance, defense against pathogens, and the maintenance of homeostasis. The lymphatic system collects excess tissue fluid and returns it to the bloodstream, while the immune system protects the body from foreign invaders.

• Lymphatic system: Includes lymphatic capillaries, collecting lymphatic vessels, lymph trunks, and lymph ducts.

• Immune system: Composed of lymphocytes, lymphoid tissue, and lymphoid organs.

The Lymphatic System

Functions

• Lymphatic vessels collect excess tissue fluid and blood proteins from loose connective tissue.

• Tissue fluid within lymphatic vessels is called lymph.

• Returns tissue fluid and blood proteins to the bloodstream by carrying fluid to great veins in the neck.

• Lymph flows only toward the heart.

Lymph Capillaries

• Located near blood capillaries; first to receive lymph.

• Receive tissue fluid from connective tissue; increased volume of tissue fluid opens minivalve flaps to allow fluid entry.

• High permeability to tissue fluid, protein molecules, bacteria, viruses, and cancer cells.

Lacteals

• Specialized lymphatic capillaries in the villi of the small intestine.

• Receive digested fats; fatty lymph is called chyle.

Collecting Lymphatic Vessels

• Have the same three tunics as blood vessels (intima, media, adventitia).

• Contain more valves than veins, helping direct lymph flow.

• Lymph flow is not aided by the heartbeat but propelled by:

  • Bulging of skeletal muscles

  • Pulsing of nearby arteries

  • Contraction of the tunica media

Lymph Nodes

• Filter the lymph; approximately 500 in the average human.

• Superficial lymph nodes: cervical, axillary, inguinal regions.

• Deep lymph nodes: tracheobronchial, aortic, iliac regions.

• Structure:

  • Fibrous capsule surrounds the node.

  • Trabeculae divide the node into segments.

  • Outer cortex and inner medulla.

  • Afferent lymphatic vessels enter at the convex side; efferent vessels exit at the hilum.

Lymph Trunks

• Formed by convergence of collecting lymphatic vessels.

• Main trunks: jugular, subclavian, bronchomediastinal, lumbar, intestinal.

Lymph Ducts

• Thoracic duct: Originates at the cisterna chyli, receives lymph from lower limbs, abdomen, and pelvis; ascends along vertebral bodies; empties into venous circulation at the junction of the left internal jugular and left subclavian veins; drains three-quarters of the body.

• Right lymphatic duct: Empties into right internal jugular and subclavian veins.

Lymphogenous Spread of Cancer

• Metastasis can occur via lymphatic vessels, especially in cancers such as breast cancer.

• Mastectomy: Surgical removal of a cancerous breast may involve lymph node removal.

The Immune System

Functions

• Recognizes specific foreign molecules (antigens).

• Destroys pathogens effectively.

• Lymphocytes are key cells of the immune system.

• Lymphoid tissue and lymphoid organs (lymph nodes, spleen, thymus, tonsils, aggregated lymphoid nodules, appendix) are essential components.

Lymphocytes

• Recognize specific foreign molecules called antigens.

• Two main classes:

  • B lymphocytes: Become plasma cells, secrete antibodies, mark cells for destruction by macrophages, respond primarily to bacteria and bacterial toxins.

  • T lymphocytes: Cytotoxic T cells attack foreign cells directly, bind to antigen-bearing cells, perforate cell membrane, signal cell to undergo apoptosis, destroy virus-infected cells and some cancer cells.

  • Natural killer (NK) cells: Do not recognize specific antigens, act when detecting a lack of "self" in a cell, cytotoxic like T cells.

Lymphocyte Activation

Origins

• All lymphocytes originate in bone marrow.

• T lymphocytes travel to the thymus gland to mature.

• B lymphocytes mature in bone marrow.

Maturation

• Lymphocytes gain the ability to recognize a unique antigen (immunocompetence).

• Travel through the bloodstream, meet and bind to specific antigens, and proliferate rapidly.

Activation

• Fully activated lymphocytes gain the ability to attack antigens.

• Antigen presentation occurs via a macrophage or dendritic cell.

Proliferation and Differentiation

• Both T and B lymphocytes produce clones of:

  • Effector lymphocytes: Respond immediately, then die.

  • Memory cells: Wait until the body encounters the antigen again; basis of acquired immunity; prevent subsequent infections of the same illness.

Effector Lymphocytes

• B cells

• Cytotoxic (CD8+) T cells

• Helper (CD4+) T cells: Cornerstone of immune system, secrete cytokines, stimulate proliferation of activated B cells, cytotoxic T cells, and macrophages, amplify and fine-tune immune response.

• Targeted by human immunodeficiency virus (HIV), leading to acquired immunodeficiency syndrome (AIDS).

Lymphoid Tissue

• Mainly made of reticular connective tissue.

• Two general locations:

  • Mucous membranes of digestive, urinary, respiratory, and reproductive tracts (Mucosa associated lymphoid tissue, MALT).

  • Lymphoid organs (except thymus).

• Lymphocytes reside, activate, and proliferate here; memory lymphocytes are generated.

MALT

• Abundant in walls of intestines.

• Lymphoid follicles (nodules) and aggregated lymphoid nodules (Peyer's patches) in the distal part of the small intestine.

Lymphoid Organs

Classification

Primary Lymphoid Organs	Secondary Lymphoid Organs
Bone marrow Thymus	Lymph nodes Spleen Tonsils Aggregated lymphoid nodules Appendix

• Designed to gather and destroy infectious microorganisms and to store lymphocytes.

Bone Marrow

• Reticular connective tissue with blood sinusoids.

• Mature blood cells enter the bloodstream through endothelial cells of sinusoids.

• All lymphocytes originate in bone marrow; B cells mature there.

Thymus

• Secretes thymic hormones; most active in childhood.

• Functional tissue atrophies with age.

• Immature lymphocytes develop into T lymphocytes.

• Structure: outer cortex, inner medulla, thymic corpuscles (Hassal's corpuscles).

• Differs from other lymphoid organs: functions strictly in lymphocyte maturation, arises from epithelial tissue.

Lymph Nodes (Structure)

• Cortex: Lymphoid follicles, most B cells, T cells.

• Medulla: Medullary cords, T, B, and plasma cells.

• Lymph percolates through lymph sinuses.

• Most antigenic challenges occur in lymph nodes; antigens destroyed and activate B and T lymphocytes.

Spleen

• Largest aggregation of lymphoid tissue; blood reservoir.

• Located on the left at the level of ribs 9-11, under the diaphragm.

• Two main blood-cleansing functions:

  • Removal of bloodborne antigens

  • Removal and destruction of old or defective blood cells

• Storage of RBCs and platelets; site of hematopoiesis in the fetus.

• Large splenic artery and vein enter and exit through the hilum.

Spleen Histology

• Capsule and trabeculae provide structure.

• Splenic pulp:

  • White pulp: Thick sleeves of lymphoid tissue, destroys bloodborne antigens, provides immune function.

  • Red pulp: Surrounds white pulp, contains splenic sinusoids and cords, rich in macrophages, responsible for disposing of worn-out RBCs.

Tonsils

• Simplest lymphoid organs; masses of lymphoid tissue.

• Four groups: palatine, lingual, pharyngeal, and tubal tonsils.

• Arranged in a ring to gather and remove pathogens; underlying lamina propria consists of MALT.

Palatine Tonsil

• Often referred to as "the tonsils"; located on each side of the oropharynx between the palatine arches.

• Usual site of tonsillectomy (tonsil removal).

Lingual Tonsil

• Located on the posterior part of the tongue.

Pharyngeal Tonsil

• Commonly called the "adenoid" when enlarged; adenoidectomy is adenoid removal.

• Located in the mucous membrane of the roof and posterior wall of the nasopharynx; destroys entering pathogens in inhaled air.

Tubal Tonsils

• Posterior to the opening of the pharyngotympanic tube; provides some protection from infection to the tube.

Pharyngeal Lymphatic (Tonsillar) Ring

• Lingual tonsil: Antero-inferior part of the ring.

• Palatine & Tubal tonsils: Lateral parts of the ring.

• Pharyngeal tonsil: Posterior and superior parts.

Vermiform Appendix

• Vermiform means "worm-shaped".

• Tubular offshoot of the cecum; rich in lymphoid tissue.

Summary Table: Lymphoid Organs and Their Functions

Organ	Location	Main Function
Bone Marrow	Medullary cavities of bones	Lymphocyte origin and B cell maturation
Thymus	Superior mediastinum	T cell maturation
Lymph Nodes	Throughout body (cervical, axillary, inguinal, etc.)	Filter lymph, immune activation
Spleen	Left upper abdomen	Blood filtration, immune response, RBC/platelet storage
Tonsils	Pharynx	Pathogen removal from air/food
Appendix	Cecum	Immune surveillance, lymphoid tissue

Key Terms and Concepts

• Lymph: Fluid collected by lymphatic vessels from tissues.

• Antigen: Any molecule that induces an immune response.

• Immunocompetence: Ability of lymphocytes to recognize specific antigens.

• Effector lymphocytes: Cells that respond immediately to antigens.

• Memory cells: Lymphocytes that provide long-term immunity.

• MALT: Mucosa associated lymphoid tissue, found in mucous membranes.

Relevant Equations

• Fluid Movement (Starling's Law):

Net filtration pressure (NFP) determines movement of fluid between capillaries and tissues:

$NFP = (P_c - P_i) - (\pi_c - \pi_i)$

• $P_c$ = capillary hydrostatic pressure

• $P_i$ = interstitial hydrostatic pressure

• $\pi_c$ = capillary oncotic pressure

• $\pi_i$ = interstitial oncotic pressure

Additional info: Starling's Law is relevant for understanding how excess tissue fluid is formed and collected by lymphatic capillaries.